Generally, a pilot or control valve in a pressure control system with a main shut-off valve has its inlet port connected to one side of the main valve and an exhaust line connected to the other side. For example, with a pilot valve controlling the jacket pressure of a main valve, the inlet port of the pilot valve is connected to the upstream side in the normal direction of flow and a dumping valve for evacuating the jacket is connected to the normal downstream side. However, in the event of pressure reversal this system becomes inoperative and, where pressure reversal is occasioned, bi-directional control requires the provision of two parallel control valves, manually switching devices or the like.